The ubiquitous use of a.c. induction motors in major appliances and air conditioning equipment is well known. The simple, inexpensive design of such motors makes their choice very cost-effective from the designers point-of-view.
Low cost fractional horsepower induction motors are also known to waste a lot of electric energy as heat and to have not particularly high operating efficiency, especially when they operate at less than full load. It is not unusual for a common induction motor to draw nearly as much line current amperage when unloaded as what it consumes under full load. Although the unloaded power factor is lower, due to the lagging current draw due to the highly inductive load characteristic, with the result that some of the apparent wasted power is "returned" to the power line by the action of the back e.m.f. of the motor, a good portion of the loss persists as a combination of eddy current heating loss brought on by relatively high magnetic field densities, combined with copper losses due to the choice of minimal wire size in the windings. These loss factors are mostly brought on by cost reduction practices of the so-called modern motor, wherein a very high temperature rise can be tolerated due to improvements made in insulating materials over the past number of years.
A motor's temperature rise is a sure sign of electrical inefficiency. All the electrical losses convert directly into heat. It is not unusual for ordinary induction motors to operate with a surface temperature of 150 to well over 200 degrees Fahrenheit. Motors made with class A insulation are commonly rated for 50 degree centigrade temperature rise above ambient, while class B insulation permits a 75 degree centigrade temperature rise. Thus a lot of energy is intentionally thrown away as thermal loss in an effort to produce a cheap product.
In most induction motor applications in major appliances, the load driven by the motor varies over a wide range. A common washing machine is often equipped with a 1/2 horsepower motor, such as a General Electric Company model 54KH46JR15S which draws about 7.9 amperes under full load, and with a power factor of about 85%. Since one horsepower represents 745 watts (550 ft. lb. per second), this represents a full load efficiency of merely: EQU ((745W/2)/(115V.times.7.9A.times.0.85PF)).times.100=48% efficiency.
As a result of this, about half of the electricity consumed by the motor under full load is thrown away as wasted heat: EQU (115V.times.7.9A.times.0.85PF)-(745W/2)=400 watts wasted,
and the manufacturer admits of a 70 degree centigrade temperature rise above ambient (e.g., about 95 degrees centigrade operating temperature).
It is also known that full load motor power is seldom required in the usual washing machine. With a heavy load of clothes, the spin dry cycle and the agitate cycle may draw nearly full power: but when washing lighter loads, nowhere near the full amount of available motor power is needed. During the pump-out cycle very little power is needed, and it is also noteworthy that during the spin dry cycle the most driving torque and therefore the highest power demand occurs during the start-up of the cycle.
The result of the variable load presented by the washing machine's ordinary operation is that the motor drive seldom operates anywhere near its full capacity. However, low-cost design practices dictate that the motor torque rating must be sized for the worst case load in order to have sufficient reserve power to run the machine without stalling.
When an induction motor is operating at less than full-load, its actual electrical efficiency is miserable. For example, the mentioned General Electric 5KH46JR15S motor continues to draw nearly 5.7 amperes under NO-LOAD conditions (e.g., load disconnected from motor shaft), which simply means that over 655 apparent watts are thrown away to do nothing! The irony is, horrible dictu, the less the motor "works", the more power the motor "wastes". Even if a power factor of as low as 64% is allowed, the wasted power 655W.times.0.64F=420 watts is still very high. Under no-load, practical motors seem to assume a power factor of about 70% to 80% due to eddy current loss and copper (resistance) loss. Furthermore, motors with aluminum windings suffer somewhat more loss (but "better" power factor) than those with copper windings.
The National Electric Code has given a listing of nominal load currents which may be expected to be drawn by a.c. motors running under full load, as:
______________________________________ Single-Phase 3-Phase AC Horsepower 115 V 230 V 110 V 208 V 277 V ______________________________________ 1/6 4.4 A 2.2 A 1/4 5.8 A 2.9 A 1/3 7.2 A 3.6 A 1/2 9.8 A 4.9 A 4.0 A 2.1 A 1.6 A 3/4 13.8 A 6.9 A 5.6 A 2.9 A 2.2 A 1 16.0 A 8.0 A 7.0 A 3.3 A 2.8 A 11/2 20.0 A 10.0 A 10.0 A 4.7 A 3.9 A 2 24.0 A 12.0 A 13.0 A 6.1 A 5.2 A 3 34.0 A 17.0 A 9.5 A 7.1 A 5 56.0 A 28.0 A 15.9 A 11.9 A 10 50.0 A 28.5 A 21.4 A ______________________________________ From REFERENCE DATA FOR RADIO ENGINEERS, 5th Edition, Howard W. Sams & Co.; Library of Congress No. 4314665; page 41-13.
The principal causes for energy loss brought on by wasteful motor heating is eddy current loss in the magnetic path and "copper" loss in the windings. Cheap motors often use aluminum windings (instead of copper), and the result is even worse winding loss ("aluminum" loss!?), due to the somewhat higher resistivity of the aluminum wire. Designers often run the flux densities in the magnetic structure (particularly the stator) near saturation in order to obtain a desired level of performance with the least amount of material, and in a smaller and lighter configuration. High flux densities merely serve to aggravate heating caused by eddy current losses, and of course increased temperature aggravates the winding losses.
In the mentioned General Electric 5KH46JR15S motor, such losses due to near-saturation of the magnetic structure is very evident when the operating voltage is reduced. Yes! an unloaded induction motor can be run at much less than full rated voltage AFTER it starts, and it will continue to spin at near full speed. This particular motor runs well at 50% applied voltage under light load: and the fully unloaded (nothing connected to the shaft) current draw dips to merely about 2.1 amperes (with about 60 volts applied to the RUN winding, which is equivalent to about 1.1 amperes at full 115 line volts). Thus the wasted power becomes only about 126 watts (or merely 88 watts for a 70% PF) under no-load, whereas when it is run at the nameplate voltage the wasted power soars to about 458 watts (with 85% power factor), representing a whopping 520 percent increase in wasted energy. In this particular motor embodiment, real power waste begins to soar when the applied voltage reaches about 85 volts or so. The no load current quickly zooms up to the 5.7 ampere level when the applied voltage is increased above 90 volts up to the rated 115 volts. With 125 volts applied, the no load current jumps up to 7.3 amperes, or about 640 watts (with 70% power factor) as wasted energy! This does not mean that the motor should be run at reduced voltage except when the load is reduced, i.e. the applied motor power should be matched with the mechanical loading demand placed on the motor. It is well known that the power that an a.c. motor develops varies directly with the square of the applied voltage, and therefore a reduction to 85 volts results in a considerable reduction in available torque to be only about half that available with 115 volts applied: but even that is usually more than enough running torque for many portions of an appliance's operating cycle. Conversely, during a full-load operation of the appliance full voltage must definitely be applied, in order to avert stalling and possible burn-out of the motor. Similar performance is found with the Kenmore (Sears Roebuck & Co.) washing machine motor model C68PXDBZ3290 (part no. 62556) which is rated for 1/2 horsepower with a full-load current of 9.8 amperes (e.g., over 900 watts with an 80% power factor). It also appears that the =newer" a major appliancemotor design is, the more wasteful of energy it becomes when only lightly loaded.
A clothes dryer motor such as the General Electric Company model 5KH47ER150X which is rated 1/3 horsepower draws about 6.4 amperes from the 115 volt a.c. mains, hence running with about 34% efficiency when fully loaded. The same motor also continues to draw nearly 6 amperes under no load, and therefore (even with a 70% power factor), "wastes" over 480 watts of energy. It is also obvious that in a clothes dryer application, the load difference between that of drying a heavy wet blanket or two, and a different lighter load consisting of no more than a few pieces of lingerie demands considerably different torque from the drive motor. As in the earlier mentioned washing machine motor, this motor wastes significantly more energy when lightly loaded. This motor operates satisfactorily with only about 85 volts applied, albeit with less torque, and under this reduced voltage condition it draws only about 3 amperes (178 watts with 70% power factor) under no load (or a very light load), while with 60 volts applied it draws merely 2.2 amperes (92 watts with 70% power factor).
What is now shown is that the washing machine using a General Electric 5KH46JR15S or 5KH47KR223B (or equivalent) motor and the clothes dryer using a motor like the General Electric model 5KH47ER150X represent a typical laundry combination which together can readily "waste" about a kilowatt of electric power when running with less than full load. Taking into consideration that the typical laundary machine may operate for 25 to 60 minutes per day on the average, the overall amount of energy merely wasted by the cummulative energy consumption of the hundreds of millions of laundry machine operation performed each day is enormous.
The continuous duty rated Dayton model 5K461B motor produces 3/4 horsepower and is intended for two-speed belt-driven exhaust fan applications. From this table of measured performance you will see that a lot of energy can be saved by reducing the motor voltage when the load is light.
______________________________________ Applied RUN Line Current Amps (Watts) Output Winding High/Low Operating Speed Loading Volts 1,725 RPM 1,140 RPM Condition ______________________________________ 115 11.0 A 1,075 W) 7.4 A 723 W) Full 115 8.2 A (660 W) 7.0 A (563 W) None 85 5.0 A (297 W) 4.5 A (267 W) None 60 3.2 A (134 W) 3.0 A (126 W) None ______________________________________ Assumed Power Factor: 85% full load, 70% no load.
Hence there is a whopping 802 percent difference in the amount of energy consumed (at 1,725 RPM) between full load at 115 volts and no-load, which can run just as well at 60 volts (half-voltage). More importantly, the reduction in wasted energy obtainable with no-load (e.g., light load) on the motor between the condition of full excitation and partial excitation is nearly 500 percent!
Another wasteful appliance is the common dishwasher. A 1/3 horsepower motor is commonly used, but the torque developed by the motor is only needed during short portions of each running cycle. Since the typical dishwasher motor, such as the Emerson Electric Co. industry standard type 4093, sold by the W. W. Grainger Company as their model 4K180, draws about 6.5 amperes at 3,450 r.p.m. under full load about 748 watts (or 598 watts for an 80% PF) are consumed while running with about 41% efficiency. Worse however is the consumption of about 375 watts (with 70% power factor) under no load. This wasteful performance continues throughout the machine's operating cycle even when little motor torque is needed, such as during pump-out and light-load washing.
Refrigeration equipment, and in particular air-conditioners, operate over a rather wide load range, depending upon ambient temperatures, humidity and so forth. The typical sealed refrigeration compressor unit employs an integral 1/4 horsepower induction motor running at 3,450 r.p.m. and drawing about 4.0 amperes. The Kelvinator model A044-1 is such a combined 1,050 B.T.U. rated motor and compressor unit. The resulting full-load efficiency with 80% power factor is: EQU ((745W/4)/(4.0A.times.117V.times.0.8PF)).times.100=49.8% motor efficiency.
The lost power under full load is: EQU (4.0A.times.117V.times.0.8PF)-(745W/4)=188 watts.
It is well known that a refrigeration compressor works harder under some conditions of ambient humidity and temperature than what it does under other conditions. The compressor motor is again sized for the worst case condition to minimize the liklihood for stalling, while much of the time the motor is actually working a lesser load. The same kinds of light load inefficiencies occur in the compressor motor as were mentioned for the washing machine motor, because cheap design practices prevail and thus high magnetic flux densities and high winding current densities are allowed by the manufacturer.
Reducing the motor voltage as the compressor load decreases can save considerable energy and it can also reduce unecessary hum noise produced by the motor and as a result a more quiet and more energy efficient product results.
Reduction of the magnetic flux density in the induction motor's stator has a two-fold effect. Eddy current losses are reduced, and the inductance of the windings increases somewhat, when the magnetic path flux density is decreased. The combined effect is an increase in the impedance of the winding and a resultant lowering of current through the winding. Flux density can be reduced by increasing the amount of iron in the magnetic path, or by increasing the winding turns. For a given motor design, however the only practical method for reducing the flux density is to reduce the current flow through the winding, such as by lowering the terminal voltage. Such a simple-minded approach usually merely results in lowered motor performance and probable stalling, unless the load is also much reduced. For this reason, common motor design practice intentionally tolerates wasting a lot of energy as heat build-up in order to produce an economical, essentially cheap motorized product which has sufficient reserve horsepower or running torque to operate under the worst case load scenario. The result is that the consumer pays out in wasted electricity more than what is saved by not having a more energy efficient motor running the appliance in the first place. The most far-reaching and probably most devastating fact, however, is the enormous waste of non-renewable natural resources this usual kind of cheap and low efficiency motor engineering incurs.